Add RouteListingPreference getter and listener to MediaRouter2

This change enables MediaRouter2 instances to get updates for any
RouteListingPreference changes in the target app. The CL also exposes a
new getter for currently set RouteListingPreference.

Test: atest CtsMediaBetterTogetherTestCases
Bug: 192657812
Change-Id: I0dd292850600602eb972b701e64c944ea01c4a17